In ink-jet printing tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops image-wise onto the ink-receiver. Printing can be accomplished by moving a print head across the ink-receiver or vice versa. Early patents on ink-jet printers include U.S. Pat. No. 3,739,393 (MEAD CORP), U.S. Pat. No. 3,805,273 (MEAD CORP) and U.S. Pat. No. 3,891,121 (MEAD CORP).
The jetting of the ink droplets can be performed in several different ways. In a first type of process called continuous ink-jet printing, the ink stream jetted from an orifice of the print head is broken up, by applying a pressure wave pattern to this orifice, into ink droplets of uniform size and spacing, which can be electrostatically charged or not as desired. In one embodiment the charged drops are deflected by an electric field into a gutter for recuperation, while the uncharged drops are undeflected and land on the ink-receiver to form an image. In an alternative embodiment it is the charged droplets which land on the ink-receiver to form an image and it is the uncharged droplets which are recuperated.
According to a second process the ink droplets can be created by a “drop on demand” method (DOD). A drop-on-demand device ejects ink droplets only when they are needed for imaging on the ink-receiver, thereby avoiding the complexity of drop charging, deflection hardware, and ink collection. In drop-on-demand ink-jet printing, the ink droplet can be formed by means of a pressure wave created by a mechanical motion of a piezoelectric transducer (so-called “piezo method”), or by means of discrete thermal pushes (so-called “bubble jet” method, or “thermal jet” method).
It will be readily understood that the optimal composition of an ink is dependent on the ink jetting method used and on the nature of the ink-receiver to be printed.
The ink compositions can be roughly divided into:                water based, the drying mechanism involving absorption, penetration and evaporation;        oil based, the drying involving absorption and penetration;        solvent based, the drying mechanism involving primarily evaporation;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification;        UV-curable, in which drying is replaced by polymerization.        
In most applications of inkjet printing the printed image is the final product at the disposal of the end-user. This is for instance the case in so-called small office/home office applications whereby information digitally stored on a personal computer is printed on a small format and on a small number of copies. This includes, for example, printing of text and images for an article, a report, a presentation, and the printing of photographs taken by a digital camera. Other end-user applications include wide format poster printing, proofing printing, and industrial applications, such as the printing of packaging elements, cables etc.
However, several applications are known wherein the jetted image is not the final product but an intermediate or mask for the image-wise exposure of a UV sensitive element. For instance, screened dot and line images prepared by ink-jet can be used for the exposure of a pre-sensitized offset plate precursor, a flexographic printing plate precursor, a silk screen printing element precursor, and a printed circuit board precursor.
For instance, EP 992846 A (AGFA) discloses a method for the production of a printing plate by exposure through a mask consisting of a screened ink-jet image formed on an ink-jet receiver comprising an N-containing polymer as binder.
EP 1349004 A (AGFA) discloses a method for the preparation of a flexographic printing plate involving the following steps, in order:
(1) preparing an ink-jet recording material comprising (i) a transparent support having front and back sides, and (ii) at the front side a layer assemblage of at least two ink receiving layers, comprising a binder, a cationic mordant in at least one of these layers, and further a spacing agent in the top layer of said assemblage,(2) jetting information-wise, according to digitally stored data, droplets of an UV-absorbing ink onto the front side of said ink-jet recording material by means of an ink-jet printer, thus forming a screened printed ink-jet image,(3) flood exposing by actinic light a flexographic printing plate precursor through a master consisting of the ink-jet image obtained by steps 1 and 2,(4) developing the exposed flexographic printing plate precursor into a flexographic printing plate.
Further patents disclosing methods and specific ink compositions for jetting onto a transparent material which are then used as a photo mask include U.S. Pat. No. 5,495,803 (GERBER), EP 737722 A (JUJo PAPER) and WO 9725206 (POLYFIBRON TECHNOLOGIES).
U.S. Pat. No. 3,531,479 (KODAK) discloses light stable compounds suitable as colorants and UV inhibitors for various plastics.
U.S. Pat. No. 5,489,503 (CIBA) discloses UV absorbers of the bis- or tris-2′-hydroxyphenyltriazine type for use in photographic materials, inks and recording materials for ink-jet printing, and in surface coatings.
GB 1096488 (BASF) discloses pyrazol-3-one compounds for mass colouring of styrene polymers.
U.S. Pat. No. 5,997,628 (ENGELHARD CORP) discloses a heat stable red shade yellow pigment composition.
In order to obtain a good differentiation between image and non-image parts in the final print, it is indispensable that the ink-jet image serving as a mask shows a high density in the ultra-violet spectral region. This requires the incorporation in the ink-jet printing ink of an efficient ultra-violet absorbing compound.
The incorporation of particular classes of UV-absorbers in ink-jet inks is well known in the prior art. References on the subject include U.S. Pat. No. 4,256,493 (DAINIPPON TORYO) disclosing p-benzophenones as UV-absorbers; JP 58049764 A disclosing 2,2′-mercaptopyridine-N-oxide sodium salt as UV-absorber; JP 59053566 A disclosing an ink containing 2-hydroxy-4-methoxybenzophenone-5-sulphonic acid; Research Disclosure 242 284-5 (1984) disclosing benzophenones, benzotriazoles, benzylidene malonates, salicylates, monobenzoates, oxamides and other compounds suitable as UV-absorbers.
However, there is a permanent need for still more efficient UV-absorbers in ink-jet inks.